Fusion of weakly bound nuclei

The effect of the break-up of weakly bound nuclei on the fusion cross section is a subject of major interest, since beams of unstable nuclei with such characteristics, especially halo nuclei, have become available. The understanding of this effect is important for the production of nuclei near the neutron drip line and super-heavy nuclei. However, although there has been many experimental and theoretical works in this field, at the present it is still far from being understood. A full understanding of the fusion and break-up processes induced by stable weakly bound projectiles is an essential step for the study of the fusion induced by radioactive beams. For instance, the coupling between the relative motion and intrinsic degrees of freedom of such interacting nuclei may enhance the fusion cross section relative to one-dimensional potential predictions, or the break-up of the weakly bound nucleus before reaching the fusion barrier may inhibit it by the absorption of flux that would otherwise go to fusion. In order to have a comprehensive picture of this problem, one has to study different energy regimes, from the sub-barrier to above barrier energies and also one has to span different nuclear masses, in order to investigate the effect, on the fusion, of nuclear and Coulomb break-ups. The suitable stable nuclei for this kind of study are 9Be, 6Li and 7Li, due to their small separation energies that should favor the break-up process. In this talk I will give an overall picture of our present understanding of the field, based mostly on recent experiments performed by our group and collaborators, on many different systems involving 9Be, 6,7Li, 6He and targets ranging from 27Al to 209Bi, and other systems involving tightly bound projectiles on the same targets and/or leading to the same compound nuclei Fusion-evaporation and fusion-fission channels are measured. Special emphasis will be given to very recent and still unpublished data on complete and incomplete fusion of 9Be+144Sm, measured at Buenos Aires by the detection of delayed KX-rays, to the first experiments with radioactive beams performed in the South Hemisphere (Sao Paulo) on the elastic scattering of 6He by 27Al and to the discovery of a new manifestation of the dispersion relation, named by us as Breakup Threshold Anomaly (BTA). All those results might be published as letters before October 2005.